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Re: [Xen-users] How to share data between guest domains

  • To: xen-users@xxxxxxxxxxxxxxxxxxx
  • From: Brock Edward Palen <brockp@xxxxxxxxx>
  • Date: Sun, 19 Mar 2006 14:14:12 -0500
  • Delivery-date: Sun, 19 Mar 2006 19:08:38 +0000
  • List-id: Xen user discussion <xen-users.lists.xensource.com>

Todd D. Esposito wrote:

On Sun, March 19, 2006 5:59, Richard Jones said:
On Sat, Mar 18, 2006 at 09:00:28PM -0600, Todd D. Esposito wrote:
However, on that note, I wonder if you could mount the same file system,
say something like /usr, into multiple domU's READ ONLY.  That could
certainly save some trouble when updating applications, etc.  Anyone
This would only work if the disk is read-only *and* never changes
while the machines are up.  The reason for that is that the kernel
keeps disk blocks cached in memory, and assumes that the disk won't
change randomly underneath.  If they did, then it would lead to a
situation where the kernel is randomly using a mix of stale (cached)
and fresh blocks.

In other words, if you wanted to upgrade parts of /usr, you'd need to
shut down all your domUs, carry out the upgrade operation, and then
boot them up again.

This is in line with my first instinct, but I'm one of those "try it
anyway, it might work" kinda guys, so thanks for saving me some lab time.

Over here all our domUs have separate partitions.  You only need max
3G for a complete Debian with plenty of packages, and how much does 3G
of disk cost these days?  The task then moved to automating sysadmin
as far as possible -- see cfengine and apt-cron.

Sure, disk is cheap; my aim was really to reduce sysadmin time/overhead. I've glanced over cfgengine, and will likely go that way as my number of
systems (and domU/system) grows.  apt-cron won't do it for me, as I'm on
Gentoo, but there are no shortage of similar tools.  Thanks for the

- Todd
I use xen with gentoo also one thing I do and i know is documented in some gentoo doc's is i only use one portage tree I NFS export /usr/portage read-only and /usr/portage/distfiles rw to all my dom'U's from dom0. When i sync I sync dom0 like normal
emerge --sync
and the domU's i
emerge --metadata

This only saves about 1G/domU but its nifty to only sync once from rsync.gentoo. I used to use a local rsync mirror

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